CREL EXPERIMENTAL FACILITIES

Transcription

1 CREL EXPERIMENTAL FACILITIES Most systems of interest are multiphase and opaque and, hence, special experimental techniques are needed to determine the flow pattern, mixing and phase distribution. We enclose here only a brief description of the available unique experimental facilities at CREL in order to encourage our sponsors to use them on joint projects or for contract work. The main facilities consist of: Note: More detailed information is available upon request (angela@wuche.wustl.edu). Computer Automated Radioactive Particle Tracking (CARPT) This is a unique facility for monitoring velocity profiles and turbulent parameters of solids and/or liquids in gas-liquid, gas-solid, liquid-solid and gas-liquid-solid systems. CARPT is used for model verification, CFD validation, cold modeling, scale-up, evaluation of distributors and column internals on flow profiles. Computer automated radioactive particle tracking (CARPT) CARPT electronics and high precision calibration device 21

2 Computed Tomographic Scanner (CT) This is a unique facility for evaluation of three dimensional density profiles in composites and in three phase reactors. CT is used in cold modeling, scale-up, examination of the effect of internals, etc. Computed tomography (CT) New portable CT unit Highlight: CARPT/CT facilities are unique for studying the hydrodynamics of opaque multiphase systems which is not accessible by other means. Data collected via CARPT-CT is used for validation of multiphase CFD codes for flow pattern and mixing determination. No other laboratory in the world has such a combination. These experimental systems are versatile in the sense that they are capable of providing experimental data over a wide range of operating conditions, and they provide data not only on the local scale but also over the entire domain of the flow. Together the two techniques are capable of acquiring information for the complete description of the flow in a time average sense, with CARPT providing instantaneous velocities as well. In addition, these systems have been extended for measurements at high pressure and temperatures. Only CARPT/CT have the capabilities of providing data in multiphase systems at very high holdups of the dispersed phase i.e. systems that are opaque. Highlight: As the next figure illustrates, CARPT-CT can be used for development of phenomenological hydrodynamic models (e.g. bubble column example) which have the capability of predicting flow and mixing (as demonstrated by prediction of liquid and gas tracer response) in hot pilot plant units as done at the Advanced Fuels Development Unit (AFDU) in LaPorte, Texas. 22

3 Reactor model: Bubble column example MP-CARPT electronics The Figures below show the schematic of the new MP-CARPT electronics and the connections of the electronics components. The MP-CARPT unit essentially consists of detectors, formed by a photomultiplier tube (PMT) connected to the base amplifier. This base amplifier is powered by a power supply unit and the output signal from the base amplifier goes to timing filter amplifier (TFA) input for amplification. Both power supply unit and timing amplifier sit in a NIM bin. Each timing amplifier has 8 channels (one for each detector). The timing amplifier is connected to the pulse processor card (one card is required for one timing amplifier, thus 8 detectors need only one card). The pulse processor card functions as a discriminator, scaler and an interface to the PC. This pulse processor card sits in a compact PCI box and it is connected to the back plane of compact PCI which also holds a PC on a card. Same as previous CARPT setup NaI (T1) crystal+ Photomultiplier Bicron 2M2/2-x 2 x2 Power supply Canberra 3002D Photomultiplier Tube Base amplifier Canberra 2007 NaI readout module developed at ORNL (8 channels per module, needs multiple module) Collects data and performs data analysis NIM Bin (crate for power) ORNL Timing Filter Amplifier (8 channels) Multi-level window Discriminator Human interface Monitor, mouse keyboard, etc. Scaler Scaler Converts pulses to logic pulses, sorting them (10 MHz) (10 MHz) according to energy range (0.7 to 0.9 Mev, 1 to 1.2 MeV, for example) Counts pulses until read out and reset (1 scaler per energy range) Bus Interface Reads data from scalers and interfaces to pc Logic Also allows programming energy window, etc. MP-CARPT electronics PC on a card Under windows Acquisition Program C++ Compact PCI crate Compact PCI backplane 23

4 A single C++ program compiled and run by the user performs the data acquisition according to the needs of the user. Even though the list of electronics is long, it is all contained in only tow boxes/crates. This reduces the wiring connections and avoids the lengthy set-up procedures. Less number of components also cuts the costs significantly. Moreover, the new electronics is advanced, thus it is more efficient and faster in data acquisition. Detector NIM Bin Power Distribution Box Timing amplifier Power supply Compact PCI Unit Monitor/ Keyboard/ Mouse Pulse Processor card PC on a card MP-CARPT electronics components and connections Automatic Calibration Device for CARPT Experiments A fully automated calibration device was developed. CARPT experiments using the manual calibration device is generally very time-consuming, unsafe to the experimentalists, and has low accuracy. On the other hand, although the available automated calibration device can be used for 6-inch high pressure system, and modified for larger diameter columns at atmospheric conditions. It is able to move radioactive particles only in axial and angular directions. Therefore, CARPT experiments using this calibration device are generally also time consuming and labor intensive. This novel calibration device allows full automation in moving the radioactive particle in radial, axial, and angular directions. It can also be applied to different size of columns (up to 18 inch) at atmospheric pressure. Moreover, it is more reliable and accurate than the currently available manual calibration device. And it is fast (needs around 5 hours to complete more than 1000 calibration positions). Therefore, this calibration device can not only be applied to this study but also be applied to many other studies requiring CARPT experiments in our laboratory. 24

5 The figures below illustrate the components of this device. Schematic diagraph of the calibration device (Please note the drawing is not in scale) Picture of the automatic calibration device Dual Source Computer Tomography Setup The Dual Source Computer Tomography (DS-CT) system in the Chemical reaction engineering lab (CREL) has been successfully commissioned and is operational which can measure the distribution of the dynamically moving three phases. The systems consist of two fan beams (as shown in the Figure below), with two sources that can be used simultaneously to collect the projections data for image reconstruction. 25

6 The DS-CT unit utilizes a 137 Cs and 60 Co gamma ray sources. These sources can be changed as per the requirement of the experimental system. It is capable of scanning horizontal sections of experimental setups with a maximum diameter of 32 in. and height of 108 in. Scintillation detectors made of sodium iodide crystals are used for detection of the gamma ray photons. Each array consists of 15 detectors collimated to collect data long finer projections of finer thickness across the domain. It is designed to allow for a great deal of flexibility in collecting data for projections. This flexibility allows for high resolution of the image high spatial resolution of up to 1 mm for any diameter of the experimental setup. Detectors Three phase system (GLS) 1 st Gamma Ray source 2 nd Gamma Ray source Location of experimental setup to be scanned Optical Probes for Bubble Dynamics Measurements Four points optical probe and its data acquisition system have been acquired from Delft University and modified to measure bubble size distribution and bubble rise velocity in bubble/slurry bubble columns, gas-solid systems and a mixture of solvent and supercritical carbon dioxide called expanded bed. Four units have been developed and implemented in our laboratory. 4-point optical probe, CCD camera and their data acquisition systems 26

7 Dynamic Pressure Transducers Low to high pressure differential pressure and absolute pressure transducers (Validyne) and data acquisition systems have been acquired and used for pressure fluctuations measurements and for obtaining overall gas holdup via pressure difference measurements in bubble/slurry columns. The range of the pressure measurements of these transducers can be adjusted over a wide range. Heat Transfer Probe (a) (b) a. Picture of the heat transfer measurement probe b. Schematic diagram of heat transfer probe 1: Teflon tube, 2: brass shell, 3: heat flux sensor, 4: heater, 5: Teflon cap. The heat transfer probe, manufactured at Washington University, was a modified version of the probe developed by Li and Prakash (1997). The diameter and the length of the brass shell are 11.4 mm and 38 mm separately. The heat flux sensor (11mm 14mm 0.08mm) used on the probe is from RDF Corporation (No ), and measures both the local heat flux and the surface temperature of the probe simultaneously. The response time of the sensor (as claimed by the manufacturer) is 0.02 s. Optical Oxygen Probe System for Mass Transfer Measurements The optical oxygen probe, a fluorescence-type sensor first developed at the TU- Hannover, Germany (Comte et al., 1995), measures the dissolved oxygen (DO) concentration in a liquid phase. Such type of probe has been further developed and commercialized by Ocean Optics, Inc. and has been acquired to be used in CREL. This optical oxygen probe technique (Figure 1) consists of optical probe (model T1000), optic fiber, light source, spectrometer, USB A/D converter, PC, and software. As shown in Figure 2, a thin film coated on the probe tip emits fluorescence at about 600 nm when irradiated at 470 nm by the light source. Increase of the DO concentration quenches the 600-nm fluorescence linearly. Therefore, DO concentration data are obtained by 27

8 measuring the fluorescence intensity with the spectrometer. With a proper reactor model, these DO measured local responses are then used to determine the oxygen gas-liquid mass transfer coefficient. 1) optical oxygen probe; 2) optical fiber; 3) light source; 4) spectrometer; 5) USB interface; 6) PC and software Optical oxygen probe system (Ocean Optics, Inc) Thin film of the fluorescent material Light from the light source (~475 nm) Reflected fluorescence going to the spectrometer (~600 nm) Schematic diagram of the optical oxygen probe (Ocean Optics, Inc) 28

9 Gaseous Tracer Technique for Gas Dynamics and Overall Mass Transfer Coefficient Measurements A gaseous tracer technique along with a convolution method was developed for precise measurement of the gas phase mixing in multiphase reactors such as (slurry) bubble columns, fluidized beds and other reactor types. This technique can be used to measure the residence time distribution of the gas phase, and to provide measurements to estimate the mixing or dispersion parameter in a reactor model. As shown in Figure 1, the unit of the technique consists of tracer injection mechanism, sampling port and line, thermal conductivity detector (TCD) and/or flame ionization detector (FID), displacement pump, and PC with data acquisition software. The tracer gas, helium for instance, is injected into the reactor by impulse or step change; and the response is measured by the TCD or FID. By performing injection and sampling at various locations, gas phase mixing information in different zones is obtained. With a convolution method and proper model assumptions, the gas mixing in the distributor plenum can be characterized, which further provides the real, non-ideal tracer input profile at the gas distributor for the reactor model. With the convolution method and a reactor model, the mixing parameter in the reactor model can be estimated by fitting the reactor model to the overall measured. Compared with the traditional use of gaseous tracer technique, this method characterizes the gas mixing in the distributor plenum zone and sampling/analytical zone, and therefore effectively eliminates errors in the reactor model parameter due to the mixing in these zones. Schematic of the gaseous tracer technique applied to a bubble column 29

10 Bubble/ Slurry Bubble Column Laboratory * Low Pressure Bubble /Slurry Bubble Column Laboratory Different acrylic column sizes (1 inch to 18 inch diameters) and distributors are available for various hydrodynamics studies using CARPT/CT, tracer methods, pressure transducers, etc. The 18 inch diameter column can be equipped with internals (16 tubes of 1 inch diameter) which simulate heat exchanger tubes. Atmospheric bubble columns * High Pressure Bubble /Slurry Bubble Column A high pressure 6 inch diameter bubble/slurry bubble column facility has been developed. The high pressure setup can be operated at pressure up to 175 psig at air superficial velocity of up to 50 cm/s. Two columns of 6 inch diameter and 9 ft height have been installed for studies of the hydrodynamics in high pressure bubble/slurry bubble columns. One column is used for conducting CARPT/CT experiments and the other is equipped with ports and windows along its height (9 ft) for probes (e.g., optical probes, conductivity probes, heat transfer probes, etc.) and pressure transducer measurements. 30

11 6-inch high pressure slurry bubble column for CARPT/CT 6-inch high pressure slurry bubble column with ports and windows * Pilot Plant Scale Set-Up for Bubble/Slurry Bubble Columns and Ebullated Beds Studies A pilot plant scale set-up consisting of an 18 inch diameter 12 foot high column has been installed to characterize the hydrodynamics of bubble/slurry bubble columns. The set-up can be operated with upflow of gas and liquid as well as for liquid as a batch and gas in continuous upflow. The facility can be operated at high capacity of liquid (up to 160 GPM) and of gas. The reactor can be utilized to study the hydrodynamics of bubble column/slurry bubble column, ebullated bed and liquidsolid fluidized bed. Measurements of gas holdup, pressure drop and pressure fluctuations, liquid mixing, bubble size distribution and bubble rise velocity, etc. can be performed. 31

12 18-inch pilot plant scale ebullated bed, bubble and slurry bubble column 2D Bubble Column 2D bubble column is available to investigate hydrodynamics using CCD imaging camera and optical probe. 2D bubble column with 4-point optical probe and CD camera 32

13 Liquid-Solid Riser Facility Six inch diameter and 9 ft high liquid-solid riser facility is available for hydrodynamics studies. CARPT/CT and other measurement techniques are utilized for such investigations. Low Pressure Side ( <80 psi) 6 P High Pressure Side ( psi) RECYCLE 9 11" 6 11" R I S H O P P E R WATER TANK P P E R PUMP P EDUCTOR Liquid-solid riser 33

14 Gas-Solid Riser A six inch diameter, 30 ft tall gas-solid riser has been installed. This is the newest addition to CREL facilities. CARPT-CT studies are being performed. Figure 1 shows a schematic diagram of the riser. Gas-solid riser New CARPT facility mounted on gassolid riser 34

15 2 Downcomer Solids Control Valve 6 Glass Riser Moveable Aluminum Tray 1 Teflon Spacers Support Structure Gas Inlet Gas-solid riser Fluidized Bed 18-inch fluidized bed reactor set-up is available below as shown Filter Thickness 1 in 22 in 5.8 ft Column Diameter in Distributor plate= porous polyethelyene sheet with a pore size of microns. Porous Distributor Plenume Plate di = 17.5 in 1.5 in 1 in 1 ft Inlet Air Sprager Sprager Size of bolt which is used to connect Distributor with Flange is 0.5 in Diameter of hole is 2/16 in no. of holes is 18 35

16 Trickle-Bed Reactor (TBR) Laboratory This laboratory consists of high pressure, atmospheric and high temperature facilities for studies of reactions, hydrodynamic parameters and catalyst wetting efficiency. The high pressure trickle-bed reactor facility can be operated up to 1000 psig and can accommodate different reactor sizes. Currently it consists of 1/2", 3/4", 1", and 2" I.D. 0.57m and 1m long high pressure trickle-bed reactors (where one of them has an optically clear section-12 inches in length), gas and liquid delivery systems, and an on-line tracer analytical unit (differential refractometer) for tracer experiments. This facility is flexible enough to perform widely different investigations from low (atmospheric) to high pressure (70 atm). High temperature operation is possible in the stainless steel reactor without the optically clear section. Liquid holdup and pressure drop are measured in situ. The facility is also equipped with the periodic operation set-up option. The atmospheric pressure trickle-bed reactors facility consist of 1 inch, 3 inch and 5-5/8 inch diameter reactors, gas and liquid delivery systems. These reactors have several pressure transducers along the bed to detect flow regime transition. Pressure drop via pressure transducer, and holdup via load cells, are measured. The unit is currently equipped with periodic operation set-up. Electrochemical and dissolution techniques are developed to measure liquid-solid mass transfer coefficient in high pressure trickle bed reactors. An additional high temperature packed-bed facility consists of 1 inch stainless steel reactor mounted in a high temperature cabinet, and temperature controller. This unit is currently used to support tracer and reaction experiments by purifying the solvents and activate the catalyst. All trickle-bed facilities are interfaced with a portable and user friendly data acquisition system. High pressure/high tempressure packed bed facility 36

17 Instrumentation and data acquisition system for high pressure/high temperature packed bed facility Flow distribution measurement system via collector for atmospheric pressure trickle bed reactors facility CREL-XYTEL Unit (XYCREL) This unit is a laboratory-scale reactor system which can be used for a variety of multiphase reaction studies. It consists of two separate systems which can be operated in parallel or combined into a single system. Each system can deliver up to five gases to the reactor zone. One system is designed for atmospheric pressure studies; the second system can handle pressures up to 60 atm. Three heated saturators are also integrated into the gas delivery lines. Virtually any reactor of choice can be inserted into the reactor zone; available equipment include two furnaces (maximum temperature 2000 F) and a liquid feed pump. Analytical equipment includes two 37

18 dual-column gas chromatographs equipped with TCD detectors (both GCs) and FID detectors (one GC). Samples are injected on-line using gas sampling valves; the configuration can be easily rearranged to permit multidimensional GC analysis. Data acquisition and control hardware is interfaced using a dedicated 386 PC running a Visual Basic Program developed specifically for the CREL-XYTEL unit. CREL-Xytel unit Instrumentation and components of CREL-Xytel unit Anaerobic Digesters Many different configurations of anaerobic digesters mixed by different means such as biogas recirculation as air-lift type bioreactor, mechanical agitation slurry recirculation and liquid recirculation. Various measurement techniques are available to characterize their performance. Anaerobic digesters inside temperature controlled cabinet 38

19 Rotating Packed Bed (RPB) Rotating gas-liquid contactor is a device in which centrifugal force is employed as an adjustable drive for flow of liquid through a porous medium countercurrently to gas which is driven by pressure difference. The rotating porous medium, or the rotating packed bed (RPB), can be viewed as centrifugal analog of conventional packed beds with, however, much higher mass transfer rates. The device is equipped with transducers to measure pressure drop across the rotor and with electrodes spaced in radial and axial direction to measure liquid holdup. High Pressure (up to 6000 psig) Slurry and Basket Reactors Autoclave (1 liter) and atmospheric/high temperature (2 liters) slurry and basket reactors systems for kinetics studies and catalyst evaluation are available. Mini Packed Bed Reactor System High pressure autoclave facility Mini-packed bed reactors set-up (5 and 50 ml) (shown below) were fabricated using titanium alloy to withstand the corrosion effect from any solid acid catalyst. Stainless steel tubing (1/8 ) was used in process lines. New gas chromatograph ($ 19,000) was procured and installed for reactor effluent analysis. The flow control for the hydrocarbon substrates and carbon dioxide includes HPLC pump and ISCO pump, respectively. The reactor is placed in an aluminum jacket. The heating fluid is circulated in the jacket to maintain the reactor at desired temperature. A backpressure regulator controls the flow from the reactor at a constant pressure. 39

20 Mini-Packed Bed Reactor System Tubular Capillary Reactor Two stainless steel capillary reactors (D=0.762 mm and D=2.159 mm), L~30 m are available in CREL. Maximum temperature is dependent on the specifics of the thermal bath in use (currently that is 200 º C). Maximum pressure depends on the stainless steel coil used as the capillary reactor and the fitting in the experimental setup. Maximum pressure for the stainless steel currently used is 10,000 psi. Gas and liquid flow rates are in cc/min and ml/min, respectively. V R =13ml V R =50ml Feeding Section Reactor Analysis Experimental set up for tubular capillary reactor 40

21 Parr Stirred Tank Reactor This Hastalloy C stirred tank reactor has volume of 25 ml, maximum pressure of 3,000 psi and heater that can provide temperature up to C. It can operate either in batch or semi-batch mode. Parr reactor Experimental set-up with Parr reactor Remspec Reaction View and High Pressure Parr Autoclave Reactor For the better reactor and the process design it is vital to have the knowledge of reactants and products inside the reactor. To monitor these systems one needs to develop in-situ spectroscopic techniques. ATR-IR spectroscopy is one of these techniques which have recently gained attention. In the ATR-IR spectroscopy the infrared beam is directed through a crystal (e.g. ZnSe, diamond) by total reflection. At each reflection an electric field is established at the interface. The evanescent wave decays exponentially in the less dense medium. In this way the totally reflected infrared beam probes the less dense medium. The energy of infrared light corresponds to the energies of the molecular vibration and rotations in the molecules and therefore the IR bands can be utilized to identify the corresponding species. For monitoring the reaction in complex multiphase systems CREL procured Remspec Reaction View (price = $ 85,000). The IR probe is designed such that it can be inserted in any multiphase reactor systems available in CREL facilities. Currently the Remspec system and high pressure autoclave reactor (price = $ 20,000 volume = 300 ml) see Figure 1, is used to study alkylation reaction catalyzed by zeolites. To withstand any corrosion effect the autoclave reactor is fabricated from Hastelloy. The high pressure in the reactor and the tubing is maintained using back pressure regulator. 41

22 Remspec react-ir Chem-BET 3000 with TPD and TPR option Chem-BET 3000 with TPD and TPR option for solid catalyst characterization was procured and tested in CREL laboratory. The equipment features five flow methods of analysis: three temperature program analyses (TPR. TPO and TPD), pulse titration and physisorption (BET surface area). Chem-BET 3000 with TPD and TPR option Ozonation Reactor Set-Up A flexible ozonation reactor set-up equipped with ozone generator for studies of waste water oxidation is available. Flow Measurement Techniques Besides CARPT/CT, CREL is equipped with various hydrodynamics measurement techniques such as, heat pulse anemometry (HPA), optical probes (reflectance and transmittance), CCD camera, heat transfer probe, tracer techniques, dynamic pressure transducers, etc. 42

23 LOR (Liquid Phase Oxidation Reactors) Laboratory The equipment for this unique laboratory has been received and awaits installation. Virtual Control Laboratory The ChE Department has developed a strategic state-of-the-art HYSYS based virtual control laboratory which is available for development of reactor control protocols. Analytical Equipment Gas Chromatographs (TCD, FID, PID and ELCD detectors) with auto sampling, Differential Refractometer, Mass Spectrometer, Atomic Absorption Spectrophotometry, UV/VIS Spectrometer, FI-IR Infrared Spectrometer, Ph meter, Dissolved Oxygen meter, Ozonator, Fume Hoods, Shaking Table, Magnetic Stirrers, High Accuracy Electronic Scale, Ovens, Refrigerator. More detailed information is available upon request (angela@wuche.wustl.edu). Virtual plant simulation on a personal computer 43

24 Model-based control and optimization 44

25 Brief Description of CARPT/CT Facility COMPUTER AUTOMATED RADIOACTIVE PARTICLE TRACKING (CARPT) In the CARPT facility a single radioactive particle of size and density designed to match the properties of the phase to be traced (solids in gas or liquid fluidized beds, liquid in gas-liquid bubble columns) is introduced into the flow. Instantaneous particle position is identified by the simultaneous monitoring of the radiation intensities received at a set of NaI detectors located strategically around the column. The technique therefore is somewhat analogous to emission tomography. For a given operating condition of the flow, the particle motion is continuously tracked for long periods of time. Pre-established calibration curves for radiation intensity versus distance for each detector are then used in a linear regression scheme to determine the position of the particle at each sampling instant. Time differentiation of this position data yields instantaneous velocities and accelerations of the particle. To infer the flow field from this, the flow domain is divided into a set of compartments and the calculated instantaneous velocities are assigned to the compartment in which the particle resides at that instant of time. Each compartment ultimately has a large number of such assignments corresponding to the data collected over the period of investigations during which the system is operated at steady state. Invoking the ergodic hypothesis an ensemble average of all such velocities in a compartment yields the average velocity for each of the compartments in the flow. The instantaneous and time averaged velocities can then be used to determine various turbulence parameters of interest. Existence of stationary state has been shown by obtaining the same ensemble average when all or part of the collected data is utilized. In addition, from the number of particle occurrence in various regions of the flow one can obtain information concerning the flow stagnancies. It is the only technique that maps the flow field over the entire domain and provides particle Lagrangian velocities. The schematic of the CARPT facility is shown in Figure. 2a. Scandium 46 at 250 µ Ci activity and a half life of 84 days is used as the source of radiation. The system incorporates 32 NaI scintillation detectors and the signals from these detectors can be sampled and processed simultaneously at frequencies up to 500 Hz. COMPUTED TOMOGRAPHIC SCANNER (CT) The tomographic scanner at CREL (Figure 2b) uses the third generation scanning configuration, in which an array of collimated detectors are arranged in an arc at the center of which is the source of radiation. The whole assembly of the detectors and the source are mounted on a gantry that is capable of being rotated about the axis of the test section through a stepper motor interfaced to a host computer. The source used is an encapsulated 100 mci Cesium-137 isotope. The encapsulation is such that it provides a fan beam subtending an angle of 40 in the horizontal plane. The source has been further collimated using a 20 x 10 x 10 cm lead brick with a central slit such that the emerging beam has a thickness of 6.5 mm at a distance of 28 cm from the source. It was necessary to use the same 2" x 2" NaI detectors that are used in the CARPT facility due to economic considerations. The arc in which the detectors were 45

26 set, has a radius of cm. This radius was necessary to accommodate test sections as large as 45 cm in diameter. Given this radius and the size of the detectors only eleven of them could be accommodated in the fan beam arc of 40. If one were to use the system in this configuration there would be only eleven chordal transmittance measurements (with the number going down as the size of the test section reduces) leading not only to a coarse spatial resolution, but also to severe aliasing according to signal sampling considerations. Using a novel approach the number of detectors in the arc was effectively increased by making use of a collimator, which for a given view moves across the detector arc, so that each detector samples multiple rays, each of which is unique. However, the price that is paid is in the form of increased scan time (the scan time increases in proportion to the number of additional rays sampled by a detector). The movement of the collimator is effected by another independent stepper motor, also interfaced with the host computer. The collimator made of lead is 6.35 cm deep and has a height of 7.62 cm so that the detectors are completely shielded by the collimator. It also has rectangular holes 5 x 10 mm at locations appropriate to each of the detectors for sampling the beams. The whole assembly weighs about 90 kgs and consequently the maximum rotational speed that can be achieved is about 1 rpm. Therefore the voidage distribution that can be obtained can only be a time averaged one. The entire scanning assembly has a central opening large enough for it to translate up and down with respect to the test section. This latter motion is effected by means of four precision square threaded screws supporting the scanning assembly at the corners. They can be driven synchronously to accomplish the axial positioning required for scanning different horizontal sections of the column. The data acquisition system for CT is the same as that for CARPT. However, the acquisition program differs in that it additionally accommodates the software for stepper motor control required for the scanning motions. The algorithm that is adopted for image reconstruction constitutes a truly unique feature of the scanner. It is conventional for scanners to use for image reconstruction either a Fourier based algorithm such as the Filtered-Back projection, or an algorithm based on the Algebraic Reconstruction Technique. However, we have demonstrated that the Estimation-Maximization algorithm based on maximum likelihood principles, results in better reconstructions than any of the others. Various sizes of bubble /slurry bubble column are used with CARPT/CT facility. THE SIGNAL PROCESSING AND DATA ACQUISITION SYSTEM FOR CARPT AND CT γ photons randomly emitted from the source get attenuated in number proportional to the void distribution that the beam encounters as it traverses the test section. Each photon entering the crystal produces a light pulse, which is sensed and amplified by a ten stage photo multiplier tube integrated with the detector. The biasing of the cathodes of the detectors (1000 Volts) is provided by two high voltage power supplies (Canberra, Model 3002D). The voltage pulses are amplified using timing filter amplifiers (Canberra 2111). The amplifiers accept positive or negative current pulses from the detectors and deliver output pulses in the range of ± 5V range. They have 46

27 independent adjustments for the differential and integral time constants in their RC- CR circuits. These controls enable one to shorten the tail of the signal pulse and to choose a suitable amplitude, respectively. The power to the amplifiers is fed by NIM/BIN power supplies (Canberra 2000). The signals from the amplifiers are fed to discriminators to eliminate undesired secondary emissions. The threshold for the discriminators is continuously adjustable from 15 mv to 1 V either by a potentiometer on the front panel or by software control. A threshold voltage of 45 mv was found to be adequate for removing most of the secondary emissions. Thus, the discriminator produces a logic pulse corresponding only to those photons depositing their full energy in the detector. The logic pulses are counted directly using a multichannel 24 bit scaler (binary counter). The scaler also carries a temporary buffer corresponding to each channel for storing the accumulated counts. A function generator inputs a sine wave at a known frequency to one of the channels of the scaler as a reference input for error control. A list sequencing crate controller with 8K FIFO (first in - first out) memory serves as a buffer when the contents of the scaler are emptied at user specified sampling rates. When the FIFO memory is half full a CAMAC (Computer Automated Measurement and Control) crate controller transfers its contents to the host computer hard disk via a GPIB (General Purpose Interface Bus (IEEE 488). The transfer from the scalar buffer to the FIFO memory, as well as the transfer from the FIFO memory to the computer memory and later to the hard disk, proceed in parallel with no data loss. The advantage of using CAMAC instead of A/D converters is that it allows a wide range of modular instruments to be interfaced to a standardized crate. The crate has a dataway that provides a pathway between the modules and a host computer. The crate has a number of stations in which different modules (such as scalers, discriminators, etc.) can be inserted. The last two stations of the crate are meant for crate controllers whose purpose is to issue commands to the modules and also transfer information between the modules and the host computer. 47

28 Schematic of the Computer Automated Radioactive Particle Tracking (CARPT) Facility 48

29 Schematic of the CT Scanner 49

30 50